1. Field of the Invention
The present invention relates generally to a power amplifier (PA) or the corresponding baseband modem design to efficiently utilize PA. More particularly, the present invention relates to a method and apparatus for reducing a Peak to Average Power Ratio (PAPR) to achieve linearization of a power amplifier.
2. Description of the Related Art
With the increasing demand not only for voice calls but also for mass data services such as various multimedia Internet services in wireless communication markets, Orthogonal Frequency Division Multiplexing (OFDM), Single Carrier Frequency Division Multiplexing (SC-FDMA), and Wideband Code Division Multiple Access (WCDMA) are attracting attention as wireless transmission technologies capable of meeting the demand. These technologies are applied to standards such as the Institute for Electrical and Electronics Engineers (IEEE) 802.16e Wireless Broadband (WiBro) or Mobile Worldwide Interoperability for Microwave Access (Mobile WiMAX) standard, the Wireless Local Area Network (Wireless LAN) standard, and the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard.
OFDM is a kind of Multi Carrier Modulation (MCM) that transmits signal information by means of each subcarrier in the frequency domain. Therefore, the signal being actually transmitted is a signal in the time domain, which has undergone Inverse Fast Fourier Transform (IFFT) on a symbol-by-symbol basis. Since a summation of many frequency components incurs high variability in amplitude, a signal having undergone IFFT has a characteristic that its signal strength is not maintained constantly. Because of this, OFDM results in a high Peak to Average Power Ratio (PAPR) at the power amplifier used for transmission, compared with other modulation schemes, thus causing a failure to use the power amplifier efficiently.
A high-PAPR input signal may deviate from a linear operating range of a power amplifier, causing distortion at an output stage of the power amplifier. In this case, to obtain the maximum output, a back-off scheme may be used, which lowers power of an input signal so as to allow the power amplifier to operate in its linear operating range. However, this back-off scheme increases power consumption as a back-off value for lowering power of an input signal increases, worsening efficiency of the power amplifier. This high-PAPR signal reduces power efficiency or requires a high-end power amplifier, undesirably increasing the hardware prices.
Therefore, a mobile communication system using MCM such as OFDM and OFDMA, requires a technology for efficiently reducing a PAPR, which occurs when a plurality of signals are mixed. Methods for reducing a PAPR may include clipping, block coding, phase adjusting, etc. The clipping technique, a typical technique for reducing a PAPR, maintains the phase of a time-domain signal having undergone IFFT and limits only the amplitude thereof below a desired threshold level. However, the clipping technique causes nonlinear distortion because it cuts off an input signal at a predetermined threshold level, and to compensate for this nonlinear distortion, this technique requires additional processing such as filtering. However, the filtering generates a peak regrowth that the signal component cut off by clipping re-grows, increasing again the PAPR disadvantageously.
Accordingly, there is a need for a more efficient technology for reducing a PAPR of an input signal for a power amplifier in a communication system such as OFDM.